An electronic device such as an IC, a LSI or a discrete semiconductor is mainly encapsulated with an epoxy resin by a transfer molding method. The transfer molding method has been usually used for such encapsulation because the transfer molding method is superior in terms of cost performance, reliability and productivity. In the majority transfer molding methods, it is general that an epoxy resin composition having a tablet shape (resin compact) is molded by putting the resin compact into a pot provided in a molding tool, compressing the resin compact by a plunger with being heat-melted, transferring the resin compact into a mold cavity, and then curing.
The epoxy resin composition contains an epoxy resin, a curing agent, a curing accelerator and an inorganic filler. The epoxy resin composition is prepared by mixing these materials which respectively have predetermined weights, heat-melting and kneading a mixture of the materials with a kneading machine, pulverizing the kneaded mixture (material) after the kneaded mixture is cooled, and then molding the pulverized mixture into the tablet shape. After preparing the epoxy resin composition, the prepared epoxy resin composition is stored in a low-temperature room (chamber). Further, it is general that the stored epoxy resin composition is brought out from the low-temperature room and maintained at room temperature until a temperature of the stored epoxy resin composition rises to room temperature before using the stored epoxy resin composition. In a case where overall process of heat-melting, kneading, cooling, pulverizing and molding the epoxy resin composition takes a long time, it is necessary to store the epoxy resin composition in the low-temperature room in which a temperature is adjusted to fall within the range of −20 to 10° C. for preventing deterioration of the epoxy resin composition. Further, it is also necessary to maintain the stored epoxy resin composition at room temperature for approximately ten-plus hours before using the stored epoxy resin composition. Furthermore, in a case where all of the epoxy resin composition is used in the process of molding (in one time), there is no problem. However, if a part of the epoxy resin composition is not used in one time, it is necessary to store the remaining epoxy resin composition in the low-temperature room again. If it is not done, fluidity of the epoxy resin composition significantly deteriorates depending on a storage term. If the stored epoxy resin composition which is not maintained at room temperature for sufficient time is used, the epoxy resin composition absorbs dew condensation water due to a temperature environment difference. This causes handling problems such as generation of voids, deterioration of curable property. For these reasons, it is difficult to reduce cost and man power for producing the resin compacts as well as to improve productivity thereof. Therefore, it is desirable to develop an epoxy resin composition that can be stored at approximate room temperature (in the range of approximately 20 to 25° C.)
In order to resolve the above problem, it is suggested to mold an improved compact with the transfer molding method. The compact is prepared by mixing a first component and a second component and then molding a mixture thereof into a tablet shape. The first component is prepared by mixing, heat-melting, kneading and then pulverizing an epoxy resin and an inorganic filler. The second component is prepared by mixing, heat-melting, kneading and then pulverizing a phenol resin, a curing accelerator and an inorganic filler. For example, see the following patent document 1. In this method, while it is possible to mold the compact, there is a case where curing reactions between the epoxy resin and the phenol resin become insufficient, which results in poor curable property, so that cracks, blisters and the like are likely to occur in the compact.
The problem of the poor curable property may be resolved by using a screw line type injection molding machine. However, this means that new equipment has to be introduced. Therefore, this approach is not realistic way.